1. Field of the Invention
The present invention relates to light guide plates and backlight modules typically used in liquid crystal displays, and particularly to a light guide plate having high-density diffusing dots.
2. Discussion of the Related Art
Backlight modules are used in liquid crystal display devices for converting linear light sources (such as cold cathode ray tubes) or point light sources (such as light emitting diodes) into surface light sources having high uniformity and brightness.
A typical backlight module includes a light source, a light guide plate, a reflection plate, a diffusion plate, and a prism sheet. The light source can be located beside an end of the light guide plate or beside two opposite ends of the light guide plate. The light source is used to emit incident light rays into the light guide plate. The light guide plate is used to guide incident light rays to exit an emission surface efficiently. The reflection plate is located below a bottom surface of the light guide plate, and is used to reflect some of the incident light rays that escape from the bottom surface back into the light guide plate. This reflection enhances the utilization ratio of the incident light rays. The diffusion plate and the prism sheet are located on the emission surface of the light guide plate in that order, and are used to improve uniformity of the emitted light rays.
FIG. 6 shows one example of the above-described kind of backlight module. The backlight module 10 includes a light guide plate 11, a light source 12, a side reflection sheet 14, a light diffusion layer 16, a bottom reflection sheet 15, and a curved reflection plate 17. The light guide plate 11 includes a light input surface 112, a light output surface 114 adjoining the light input surface 112, and a bottom surface 116 opposite to the light output surface 114. The light source 12 is positioned adjacent to the light input surface 112. The side reflection sheet 14 is provided on a side surface (not labeled) of the light guide plate 11 that is opposite to the light input surface 112. The light diffusion layer 16 is disposed on the light output surface 114, and the bottom reflection sheet 15 is disposed on the bottom surface 116. The curved reflection plate 17 is provided to substantially enclose the light source 12 so as to efficiently utilize light rays emitted by the light source 12.
Furthermore, a plurality of dots 118 configured for light diffusion/transmission is provided on the bottom surface 116 of the light guide plate 11. The dots 118 are formed by means of, for example, gravure printing, offset printing, screen printing and/or transfer printing. The dots 118 can have any of various predetermined shapes, such as round, square, or polygonal. The dots 118 are used to break up what would otherwise be total reflection of light rays incident at the bottom surface 116. This light diffusion ensures that the light rays exit an entire expanse of the light output surface 114 of the light guide plate 11 uniformly.
Referring to FIG. 7, a distribution of the dots 118 on the bottom surface 116 of the light guide plate 11 is shown. The dots 118 are arranged on the bottom surface 116 in a matrix, which includes a series of adjacent columns of dots 118 parallel to the light input surface 112. In a same column, a distance between the centers of two adjacent dots 118 is uniform. Each column defines a column axis that passes through the centers of the dots 118. A distance between the column axes of two adjacent columns is uniform. For example, referring to FIG. 8, a distance Ypitch between two adjacent column axes X1 and X2 is constant. Sizes of the dots 118 gradually increase with an increase in distance from the light input surface 112 along a first direction. The first direction is perpendicular to the light input surface 112 and parallel to the light output surface 114.
The dots 118 can, to a certain extent, enhance the uniformity of the light rays emitted from the light guide plate 11. Sizes of the dots 118 near the light input surface 112 are relatively small, thus a clearance between adjacent columns near the light input surface 112 is relatively large. That is, the sizes of the dots 118 are relatively small, to compensate for the close proximity of the dots 118 to the light input surface 112. However, this means relatively large areas of the bottom surface 116 corresponding to the clearances are not used for light diffusion. In this respect, the dots 118 near the light input surface 112 do not provide efficient dispersal of light rays incident at that part of the bottom surface 116. Thus, it is difficult for the light guide plate 11 to achieve a high level of uniformity of light rays emitted from the light output surface 114. Furthermore, clearances between adjacent rows of dots are substantially straight, and this tends to produce bright lines in the output light. For these reasons, the light guide plate 11 cannot necessarily provide the backlight module 10 with optimal quality of output light.
What is needed, therefore, is a light guide plate and backlight module using the light guide plate that overcome the above mentioned disadvantage.